LESSON OF THE MONTH

An unusual cause of atlanto-axial subluxation

Philip Windrum, Gary D Wright, Michael B Finch

Case reportA 55 year old woman of short stature (fig 1)presented with a four week history of paraes-thesiae in the fingers of both hands. This begansuddenly and she subsequently developed aprogressive weakness in all four limbs until shewas unable to walk or stand unaided despitebeing previously active. For the two weeksbefore presentation she had noticed an occipi-tal headache that was exacerbated by neckmovements. There was no recent trauma toaccount for the development of these symp-toms. In the past she had been diagnosed ashaving Schmid chondrometaphyseal dysplasia(CMD) in childhood, had undergone bilateraltotal hip replacements for coxa vara andrequired a caesarean section for the birth of herson. There was no known family history ofdwarfism. On examination she was of shortstature (height 4' 3") with normal facies andtrunk length but short limbs. She had athoracic scoliosis convex to the left. Neck flex-

ion produced Lhermitte’s phenomenon. Powerwas reduced in all muscle groups to grade 3/5and tone was increased symmetrically. Allreflexes were brisk with ankle clonus presentand the plantar responses were extensor.Sensory examination was normal. There wasno evidence of a peripheral arthritis. Initialinvestigations including biochemical screen,full blood picture, erythrocyte sedimentationrate, C reactive protein, thyroid function, rheu-matoid factor and autoimmune screen werenormal. Radiographs of the cervical spineshowed marked atlanto-axial subluxation(AAS) with exaggerated cervical kyphosis (fig2) while radiography of the spine and periph-eral joints confirmed the underlying diagnosisof Schmid CMD. Magnetic resonance imagingwas performed showing partial absence of theodontoid peg with anterior subluxation of C1on C2 and marked secondary cord compres-sion (fig 3). She was transferred to theneurosurgical unit where trans-oral odontoid-ectomy and posterior cervical fusion of C1 andC2 was performed. Postoperative recovery wasslow though at a recent outpatient review hersymptoms had resolved and her mobility hadreturned to normal.

DiscussionThe disorders of skeletal development includea vast array of diVering conditions althoughattempts have been made to clarify thisproblem. The “Defects of growth of the tubu-lar bones and/or the spine manifesting inchildhood”1 include the CMDs. Here there is agrowth disturbance at the metaphyses of tubu-lar bones. The epiphyses and diaphyses are lesscommonly involved and the spine can also beaVected. The CMDs can be further subdividedto include the Schmid, McKusick and Jansentypes.

Schmid first reported a mild variant of CMDin 1949.2 The condition has a prevalence of 3–4per million.3 Patients appear normal at birthand present after infancy with short stature.4

Symptoms resulting from lower limb involve-ment include pain, bony tenderness, a wad-dling gait, increased lumbar lordosis and bow-ing of the legs and may lead to confusion withrickets.5 Further confusion may result fromneurological complications such as spinal cordcompression in CMD and the proximalmyopathy seen occasionally in rickets leadingto lower limb weakness. Careful neurologicalexamination should enable a distinction to be

Figure 1 Patient with Schmid chondrometaphysealdysplasia.

Ann Rheum Dis 1999;58:679–681 679

RheumatologyDepartment, RoyalVictoria Hospital,Belfast BT9 7JBP WindrumG D WrightM B Finch

Correspondence to:Dr M B Finch.

Accepted for publication16 June 1999

made. In contrast with CMD however, ricketscan aVect the complete skeleton, has the char-acteristic radiographic changes of looser zonesand may show the typical biochemical changesof hypophosphataemia and an increased alka-line phosphatase level. Hypocalcaemia andhypocalciuria may also be noted in rickets. The

confusion has led to some patients with CMDreceiving vitamin D treatment, which, needlessto say, has no eVect.

Unlike the more severe but less commonMcKusick and Jansen CMDs, patients withSchmid CMD such as this woman have a nor-mal facies. Growth retardation and spinalinvolvement is classically said to be less markedwith an average height of 59 inches attained. Awide spectrum of disease severity does existhowever, which may account for our patient’sAAS and more severe growth retardation.

The pattern of symptoms and signs herald-ing subacute AAS in this patient was classicwith cervical pain associated with shootingpains on neck flexion, weakness and gaitdisturbance noted. She did not report anysphincter disturbance. The typical spasticweakness with hyperreflexia was evident and alack of sensory symptoms aVecting propriocep-tion and vibration sense would indicate thatthere had been sparing of the posteriorcolumns.

The condition is usually inherited as anautosomal dominant trait although late onsetand autosomal recessive forms have also beendescribed.6 7 This may account for our patient’slack of positive family history. It is caused bydefects of the collagen X gene.8–12 Collagen X isnormally expressed by chondrocytes in thedeveloping growth plates. The defect gives riseto characteristic radiological appearances withbroadened and shortened tubular bones andwidening of the metaphyses. This is most obvi-ous at the knees, resulting in genu varum orvalgum, or hips leading to coxa vara.13

In a review of 80 patients with short statureall were found to have some form of spinalinvolvement with odontoid hypoplasia leadingto AAS occurring in 6%.14 AAS itself was morelikely to occur in multiple epiphyseal dysplasia,achondroplasia and the mucopolysacchari-doses. Cases have also been associated withDown’s syndrome.15 AAS is more commonlyseen in rheumatoid disease and the seronega-tive spondyloarthropathies.16–18 In such condi-tions the mechanism of AAS diVers from thecongenital cases and results from erosion of theodontoid process and weakening of the trans-verse ligament of the atlas, which allows theodontoid process to move posteriorly from theatlas on neck flexion. Although commonly seenon radiographs in up to 25% of cases of rheu-matoid disease it less commonly leads to spinalcord compression.

Spinal involvement is usually mild in SchmidCMD with sclerosis or irregularity of the verte-bral end plates most typically seen. Althoughrare, odontoid hypoplasia and AAS have beendescribed in Schmid CMD. Therefore, allpatients of short stature with neurologicalsymptoms or signs should be investigated withradiography, computed tomography or mag-netic resonance imaging for AAS and second-ary cord compression.19 Some authors alsosuggest routine investigation in the absence ofsymptoms and even prophylactic cervicalfusion at an early age in high risk patients suchas those with the more severe CMDs.20 Patientswith short stature require specific precautions

Figure 2 Cervical spine radiograph showing atlanto-axial subluxation.

Figure 3 Magnetic resonance imaging of the spineconfirming spinal cord compression at C1/C2 level.

680 Windrum, Wright, Finch

for surgery including the cervical fusionrequired in our patient to prevent her imminentquadraplegia. Neck flexion or extension shouldbe avoided, which can be achieved by cervicalimmobilisation in a collar. The patient can alsobe intubated for surgery using a nasotrachealfibre endoscope.21 In the event of emergencysurgery the anaesthetist must presume that thepatient with short stature has an unstablecervical spine. At present the optimal surgicalmanagement of AAS is either posterior atlanto-axial fusion or fusion of the posterior axial archto the occiput.

The lessonx AAS must be suspected in patients with

abnormalities of skeletal development pre-senting with limb weakness or paraesthesiae.

x While AAS is rare in the milder SchmidCMD, it may still occur.

x Care should be taken in all patients of shortstature requiring intubation for generalanaesthesia.

x AAS may result from conditions other thanrheumatoid disease and the seronegativespondyloarthropathies.

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8 Kwan KM, Pang MK, Zhou S, Cowan SK, Knog RY,Pfordte T, et al. Abnormal compartmentalization ofcartilage matrix components in mice lacking collegen X:implications for function. J Cell Biol 1997;136:459–71.

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10 Wallis GA, Rash B, Sweetman WA, Thomas JT, Super M,Evans G, et al. Amino acid substitutions of conserved resi-dues in the carboxyl-terminal domain of the alpha 1 (X)chain of type X collagen occur in two unrelated familieswith metaphyseal chondrodysplasia type Schmid. Am JHum Genet 1994;54:169–78.

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14 Bethem D, Winter RB, Lutter L, Moe JH, Bradford DS,Lonstein JE, et al. Spinal disorders of dwarfism. Review ofthe literature and report of eighty cases. J Bone Joint Surg(Am) 1981;63A:1412–25.

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